When wanting to checkmate your opponent immediately, you are allowed to ignore an illegal move that was just made by your opponent. In this case, your checkmate stands, and you win the game. But, what if the opponent’s illegal move is making a move while his or her King remains in check. Is that particular illegal move still able to be ignored such that you can go ahead and checkmate your opponent? Or must that particular illegal move be handled differently?
Rules 11A, in particular the TD tip, and 13A answer the question.
11A. Illegal move during last ten moves. If, during a game, it is found that one of either player’s last ten moves was illegal and neither player is in time pressure (11D1), the position shall be reinstated to what it was before the illegal move. The players do not recover the time used after the illegal move. The game shall then continue by applying Rule 10, The Touched Piece, to the move replacing the illegal move. If the position cannot be reinstated, then the illegal move shall stand. Move counters on clocks that have them may be readjusted. See also 11H, Director corrects illegal move in non-sudden death
TD TIP: When the illegal move is a king left in check, special care should be taken by the director. All moves, not just the first move, in which a player’s king remains in check should be regarded as illegal. That way, an illegal move will always have occurred within the last half-move; therefore, players cannot argue that the illegal move(s) should stand. Now, go back to the first illegal move that occurred (leaving the king in check). If the moves cannot be reconstructed, go back to a position in which the king is in check, so that the king can get out of check. In complicated cases like this the director has a lot of discretion. No player should gain an unfair advantage for deliberate illegal moves, or for inadvertent ones, which were deliberately not pointed out. Remember that the clock times will not be re-adjusted; however, move counters may need to be reset to the proper move number.
So a checkmate when the king is already in check is still illegal and needs to be corrected.
13A. Checkmate. The player who checkmates the opponent’s king, providing the mating move is legal, wins the game. This immediately ends the game. See also 4A, Checkmate; 9E, Checkmate or stalemate; 12C, Responding to check; and 15H, Reporting of results. TD TIP: This means that anything that happens after the checkmate move has been legally determined (see rule 9, Determination and completion of the move) is irrelevant to the outcome of the game, including the player’s flag falling (5G).
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The key phrases here are 11A, “If, during a game…” and 13A, “This immediately ends the game.” If 13A has happened appropriately, e.g. the move is legal, then 11A cannot as it is no longer “during a game”.
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Chris, Ulmont contends that the checkmating move is illegal. But are you saying that the game is over in this case? Or are you also agreeing that since the checkmating move didn’t happen “appropriately,” the TD should correct the game?
Ken,
I believe that Chris is saying that the game is over so long as the checkmating move is itself legal whether or not the opponent’s prior move (leaving the king in check) was legal (since it wasn’t).
I’m going to defer to Chris here and take a mental note.
Then, that is handled the same as the usual case of an illegal move. The following immediate legal checkmate move stands, as usual.
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So long as the checkmating move is legal, e.g. the person playing checkmate is not the one leaving their king in check, and no claims regarding any illegal moves have been made before that, then the checkmate immediately ends the game and anything that happened before during the game, or after that, is irrelevant.